Certain soil bacteria help plants defend themselves against diseases.
In an article published in
Nature Plants, scientists show that a molecule produced by bacteria is detected by roots through a novel mechanism. This signal then triggers an immune response throughout the plant, all the way to the leaves.
Pixabay illustration image Invisible allies in the soil
Plants, like animals, possess defense mechanisms against pathogenic microbes. In the soil surrounding their roots (called the rhizosphere) live many beneficial bacteria. Some of them, particularly from the genus
Bacillus, can help plants better resist diseases.
These bacteria produce molecules called cyclic lipopeptides, known to stimulate plant immunity. They act somewhat like an alarm signal: the plant prepares itself to face potential attacks from fungi or other microbes.
However, one question remained open: how do plant cells detect these molecules?
An unexpected detection at the cell surface
A European consortium of scientists studied one of these molecules, surfactin, produced by
Bacillus bacteria. Contrary to what was thought, the plant does not detect it through a specific protein receptor.
The results, published in the journal
Nature Plants, show that surfactin inserts itself directly into the membrane of plant cells by interacting with a particular lipid called glucosylceramide.
This interaction slightly alters the membrane's structure: it causes a small physical deformation of it.
A mechanical alarm that triggers immunity
This deformation then activates mechanosensitive calcium channels, proteins capable of detecting physical deformations in the membrane. A rapid influx of calcium ions into the cell then triggers a cascade of signals.
This warning message propagates from the roots to the leaves, where the plant activates its chemical defenses. It notably produces antifungal molecules that help it fight against pathogens like
Botrytis cinerea, a fungus responsible for rot in many fruits and leaves.
A new mode of plant-microbe communication
Until now, most known mechanisms of microbe recognition by plants relied on protein receptors capable of identifying specific molecules.
This study reveals a different mechanism: here, it is the lipids of the membrane and its physical deformation that serve as a detection system.
This discovery sheds new light on the interactions between plants and soil microbes.
Understanding how beneficial bacteria stimulate plant immunity could pave the way for new agricultural strategies. By promoting the action of these microbes or the molecules they produce, it might become possible to strengthen the natural defenses of crops without resorting to pesticides.
Soil bacteria thus appear as valuable allies for more sustainable agriculture.